1. Field of the Invention
The present invention relates to a transmission/reception apparatus used for digital mobile communications using an orthogonal frequency division multiplexing (hereinafter referred to as “OFDM”) system.
2. Description of the Related Art
A Conventional OFDM transmission/reception apparatus is explained using FIG. 1 and FIG. 2. FIG. 1 is a block diagram showing an outlined configuration of a transmission system of a conventional OFDM transmission/reception apparatus. FIG. 2 is a block diagram showing an outlined configuration of a reception system of the conventional OFDM transmission/reception apparatus.
In FIG. 1, serial-parallel converter (hereinafter referred to as “S/P converter”) 11 converts a serial input signal to a plurality of parallel signals. IDFT circuit 12 performs inverse discrete Fourier transform (hereinafter referred to as “IDFT”) on the input signals. Guard interval inserter 13 inserts a guard interval for every valid symbol. D/A converter 14 performs D/A conversion on the signal with guard interval inserted.
In FIG. 2, A/D converter 15 performs A/D conversion on a reception signal. Delayer 16 delays the input signal by a valid symbol length. Correlator 17 despreads the input signal. Timing generator 18 detects the timing of the reception signal at which the correlation result becomes largest. Guard interval eliminator 19 eliminates a guard interval inserted for every symbol. DFT circuit 20 performs discrete Fourier transform (hereinafter referred to as “DFT”) on the input signal. Coherent detectors 21 to 24 perform coherent detection on the input signal. Deciders 25 to 28 judge the input signal. Parallel-serial converter (hereinafter referred to as “P/S converter”) 29 converts a plurality of parallel signals to a serial signal.
Then, the operation of the conventional OFDM transmission/reception apparatus is explained. Here, suppose the number of carriers is 4, for example.
First, the operation of the transmission system is explained using FIG. 1. A modulated signal input to the transmission system is S/P-converted by S/P converter 11. This results in four modulated signals, which are transmitted by a first, second, third and fourth carriers, respectively.
Then, the 4 modulated signals are IDFT-processed by IDFT circuit 12.
A general OFDM transmission/reception apparatus has a frame format as shown in the frame format schematic diagram in FIG. 3. That is, in a frame format used for a general OFDM transmission/reception apparatus, a signal with the same waveform as that of the last part of a valid symbol is added at the start of the valid symbol as a guard interval. The OFDM transmission/reception apparatus can eliminate a delayed signal with a shorter delay time than this guard interval through DFT processing of the reception system.
Guard interval inserter 13 inserts a guard interval into the IDFT-processed signal. The signal with the guard interval inserted is converted to an analog signal by D/A converter 14. In this way, a transmission signal is obtained.
Then, the operation of the reception system is explained using FIG. 2. The reception signal input to the reception system is converted to a digital signal by A/D converter 15.
Generally, the OFDM transmission/reception apparatus finds a correlation between a pre-DFT signal and the pre-DFT signal delayed by a valid symbol length. Then, the OFDM transmission/reception apparatus detects a DFT integration interval by detecting the timing at which the correlation result becomes largest. To be more specific, delayer 16 delays the reception signal by a valid symbol length, then correlator 17 finds a correlation and timing generator 18 detects the timing at which the correlation result becomes largest. Guard interval eliminator 19 eliminates the guard interval from the reception signal according to this detection result.
The reception signal stripped of the guard interval is DFT-processed by DFT circuit 20. This results in 4 baseband signals, which are carried by 4 carrier. The 4 baseband signals are each subjected to coherent detection by coherent detectors 21 to 24. In this way, coherent detected signals are obtained.
Here, coherent detectors 21 to 24 are explained using FIG. 4. FIG. 4 is a block diagram showing an outlined configuration of the coherent detector of the OFDM transmission/reception apparatus. Digital multipliers 41 and 42 multiply the DFT-processed signals by pilot symbols. Conjugate complex number generator 43 generates a conjugate complex number for the input signal.
In a general frame format, a pilot symbol, a known reference signal, is added before a message interval. In a general coherent detection method, a fading variation is detected using a pilot symbol.
In (nT) which is a DFT-processed input signal in a pilot symbol interval is expressed as In (nT)=P (nT) A (nT)·exp (jΘ(nT)), where P (nT) is a pilot symbol, A (nT) is an amplitude variation due to fading and exp (jΘ(nT)) is a phase variation due to fading.
F (nT), which represents a variation due to fading, is expressed as follows:
                                                                        F                ⁢                                                                  ⁢                                  (                  nT                  )                                            =                            ⁢                              In                ⁢                                                                  ⁢                                                      (                    nT                    )                                    ·                  P                                ⁢                                                                  ⁢                                  (                  nT                  )                                                                                                                        =                                ⁢                                                                            {                                              P                        ⁢                                                                                                  ⁢                                                                              (                            nT                            )                                                    ·                          A                                                ⁢                                                                                                  ⁢                                                                              (                            nT                            )                                                    ·                          exp                                                ⁢                                                                                                  ⁢                                                  (                                                      j                            ⁢                                                                                                                  ⁢                                                          Θ                              ⁡                                                              (                                nT                                )                                                                                                              )                                                                    }                                        ·                    P                                    ⁢                                                                          ⁢                                      (                    nT                    )                                                              ⁢                                                                                                                      =                            ⁢                              P                ⁢                                                                  ⁢                                                                            (                      nT                      )                                        2                                    ·                  A                                ⁢                                                                  ⁢                                                      (                    nT                    )                                    ·                  exp                                ⁢                                                                  ⁢                                  (                                      j                    ⁢                                                                                  ⁢                                          Θ                      ⁡                                              (                        nT                        )                                                                              )                                                                                        1        ⁢        ◯            Here, in a modulation system such as a QPSK modulation system in which the amplitude is constant and only the phase contains information, P (nT)2=1. Therefore, expression {circle around (1)} is expressed as follows:F(nT)=A(nT)·exp (jΘ(nT))Then, digital multiplier 41 obtains signal F (nT) that represents a variation due to fading by multiplying DFT-processed input signal (baseband signal) In (nT) by pilot symbol P (nT) in a pilot symbol interval.
Then, conjugate complex number generator 43 generates a conjugate complex number about F (nT), a signal representing a variation due to fading. In this way, conjugate complex number F (nT)* of F (nT) signal expressing a variation due to fading is obtained. Conjugate complex number generator 43 inverts the polarity of the Q component of the input signal and generates a conjugate complex number. Therefore, conjugate complex number F (nT)* is expressed in the following expression:F(nT)*=A(nT)·exp (−jΘ(nT))
Then, digital multiplier 42 multiplies the DFT-processed input signal (baseband signal) by the conjugate complex number of the signal representing a variation due to fading. In this way, a coherent detected signal is obtained.
Here, suppose the fading variation is sufficiently slow compared to the interval of pilot symbols and the fading variation is constant between pilot symbols. Based on this supposition, coherent detected signal Dout (nT) is expressed in the following expression:
                                                                                          D                  out                                ⁢                                                                  ⁢                                  (                  nT                  )                                            =                            ⁢                                                D                  in                                ⁢                                                                  ⁢                                                      (                    nT                    )                                    ·                  A                                ⁢                                                                  ⁢                                                      (                    nT                    )                                    ·                  exp                                ⁢                                                                  ⁢                                                      (                                          jΘ                      ⁡                                              (                        nT                        )                                                              )                                    ·                                                                                                                      ⁢                              A                ⁢                                                                  ⁢                                                      (                    nT                    )                                    ·                  exp                                ⁢                                                                  ⁢                                  (                                      -                                          jΘ                      ⁡                                              (                        nT                        )                                                                              )                                                                                                        =                            ⁢                                                D                  in                                ⁢                                                                  ⁢                                                      (                    nT                    )                                    ·                  A                                ⁢                                                                  ⁢                                                      (                    nT                    )                                    2                                                                                        2        ⁢        ◯            
In expression {circle around (2)}, A (nT)2 is the component with a constant phase and variable amplitude. Therefore, the phase variation of coherent detected signal Dout (nT) is only dependent on Din (nT). Therefore, the phase of the reception signal is demodulated by digital multiplier 42 multiplying the DFT-processed input signal (baseband signal) by a conjugate complex number of the signal indicating a variation due to fading. The QPSK modulation system is a modulation system with a constant amplitude and variable phase. Therefore, the OFDM transmission/reception apparatus performs coherent detection by demodulating the phase of the reception signal.
Furthermore, the OFDM transmission/reception apparatus can also eliminate any phase difference between transmission and reception carriers and phase variation by frequency offset as well as fading variation.
In a modulation system such as a 16QAM modulation system with a variable phase and variable amplitude, the OFDM transmission/reception apparatus detects a fading variation by dividing an input signal in the pilot symbol interval by a pilot symbol. The OFDM transmission/reception apparatus then performs coherent detection by dividing the input signal by a signal indicating a fading variation.
The OFDM transmission/reception apparatus can also use a delay detection system as the demodulation system.
The coherent signals detected by coherent detectors 21 to 24 are judged by Deciders 25 to 28. After judgment, the 4 signals are converted to a single signal by P/S converter 29. In this way, a demodulated signal is obtained.
As shown above, the conventional OFDM transmission/reception apparatus adds a signal with the same waveform as that of the last part of a valid symbol at the start of the valid symbol as a guard interval. By providing the guard interval, the OFDM transmission/reception apparatus can eliminate a delayed signal whose delay time is shorter than the guard interval through DFT processing of the reception system.
The explanation above referred to a case where the number of carriers is 4, but the same apparatus configuration can also be used when the number of carriers is increased to 8, 16, 32, 64, and so on.
However, the conventional OFDM transmission/reception apparatus adds a guard interval with a fixed length for every valid symbol independently of the channel quality. Therefore, in the case that the channel quality is good and the delay time of the delayed signal is short, a guard interval 2 that is longer than necessary is added, which deteriorates the transmission efficiency.